A casting system may be, for example, a strand casting system, a billet strand casting system or else a die-operated continuous casting and rolling system. In the case of a strand casting system, a strand, generally a metal strand, in particular a steel strand, is drawn off from a die by means of driven rollers or pairs of rollers. In the case of a billet strand casting system, generally a plurality of extruded billets are cast in a die, generally two to six extruded billets.
Generally, electrical driven rollers or pairs of rollers are provided for guiding a cast material, for example a strand or an extruded billet, when it is being drawn off. For example, a strand is drawn off substantially vertically out of the die and transferred into a substantially horizontal direction by means of a casting bow.
In order to reduce the effort involved in rolling in a rolling mill, in the case of casting systems the rollers or pairs of rollers are advantageously used not only for guiding the strand but also for reducing the thickness of the cast material.
Critical variables in the casting of a cast material are the casting speed and the desired final thickness, if a thickness reduction is envisaged.
The rotational speed of the drives for the rollers or the drives for the pairs of rollers may serve for setting the casting speed of the cast material. For example, the mean value of the speed of all the drives is kept constant for this purpose. As a result, the casting speed drops as the thickness of the strand is increasingly reduced. However, since the driven rollers rotate with the same radial speed, adequate allowance cannot be made for a change in the speed of a portion of the cast material that results from the thickness reduction. Therefore, in the case of such a method, generally no thickness reduction of the cast material is envisaged for this reason.
Alternatively, it may be provided that the rollers or pairs of rollers acting on the cast material are driven by means of drives which all run with the same load. The pairs of rollers together with the drive and means for producing rolling force are referred to as the reduction stand. In the case of a dynamic thickness reduction—dynamic since the rolling forces depend on the time-variable phase response within the strand—of the strand or billet, operation of the drives with the same load has the consequence that, with low vertical force or rolling force on the strand, the frictional forces are so low that the roller loses adherence and does not transfer any forward motion, or transfers reduced forward motion, to the strand. Moreover, increased friction occurs in the case of rollers with increased vertical force or increased rolling force on account of the evenly distributed load on the drives, and increased friction leads to a slowing of the circumferential speed of the roller concerned. This leads to a slowing of the speed of the strand or to the cast material coming to a standstill in the casting system.
On account of a dynamic distribution of forces in the case of roller drives operated with the same load—the thickness reduction of the strand over the various pairs of rollers is highly process-dependent and dynamic during casting—instabilities in the casting speed occur. In particular, the dynamics of the thickness reduction are determined in part by the calculated liquid core component within a strand, which is determined by appropriate models that are not the subject of this application.
Patent specification EP 0 463 203 B1 discloses a guiding method for electrical drives of rollers of a strand casting system in which the strand is drawn off out of the die of the strand casting system by the driven rollers, the drives of which are individually controlled by means of controllers, and can be reduced in its thickness. A disadvantage of this teaching is that the drives consequently cannot be controlled adequately flexibly with regard to use within casting systems with reduction stands.